obstacle circles not consistent when increasing figure size

**Describe the bug**
in many scripts e.g. PathPlanning/RRTDubins/rrt_dubins.py
the black circles change size when making the figure full screen. This is caused by the fact that the obstacle is drawn with `plt.plot(ox, oy, "ok", ms=30 * size)`, that does not scale the circle properly when the figure changes size.
probably something like
```
ax = plt.gca()
ax.add_patch(plt.Circle((ox, oy), size, color="k", fill=True))
```
would work

Start with PathPlanning/RRTDubins/rrt_dubins.py and inspect how obstacle circles are drawn, then reproduce the issue by resizing or maximizing the figure. Update the affected plotting code so obstacle circles remain consistent when the figure size changes, and verify the behavior across the other scripts that use the same pattern.

Scritto dal modello di indicizzazione a partire dal testo della issue.